Filament for incandescent lamps and method of manufacturing the same



Patented Apr. 12, 1 938 UNITED STATES,

PATENT OFFICE,

FII JAMENT FOR INCANDESCEfiT LAMPS islND METHOD OF MANUFACTURING THE Jakob Salpeter, Vienna, Austria i No Drawing.

. rial No. 124,502.

Application February 6, 1937, Se-

In Austria March 28,, 1935 6 Claims. 176-132) This invention relates to the manufacture of coiled filaments of tungsten, particularly electric incandescent lamp filaments, which are stable with regard to shape 1. e. tungsten filaments which do not become distorted and are practically v 15 mounting the filament on its supports, 1. e. outside the lamp; yet in this case to obtain a sag, which is less than the average size of the crystals had to liebetween 0.05 and 1 mm. All previous methods have been based on the funda- In carrying out this invention the filament body, which consists of drawn tungsten .wire

which, on heating by the known methods is in-- clined,to develop large overlapping crystalsprior to recrystallizationis preferably subject- 35 ed to a heat treatmenton a mandrel or core at a temperature which is such that no change in the crystalline structure takes place; The temperature of this treatment is, of course, dependent also on the material, of which the mandrel is 40 formed and for example in the case of iron or copper mandrels, the temperature should not exceed 800 to 900 C. After the filament on the mandrelhas been cut up into the required lengths and the mandrel removed, for instance 45 by acids, the filament supported on a solid base is advantageously subjected to a further heat treatment at a higher temperature-preferably between 1000 and 1200 C.-but which is still a temperature below the recrystallization temper- 50 ature of the tungsten of which the filament is made.

It is to be understood that provided the material, of whichthe mandrel is made, possesses a sufficiently high melting point, such as molyb- 55 denum, both heat treatments can be carried out mental assumption that a coiled filament sags in one stage on the mandrel. The preliminary heat treatment just described is not essential but is preferred since it serves a dual purpose in that it relieves the filament wire of strain and renders the specific electrical resistance of the wire more 5 uniform throughout. After subjecting the filament to this preliminary treatment it is attached to the supporting stem of the lamp and according to this invention it is subjected for a very short time (preferably less than about of a second) to a shock heat treatment at a temperature in the neighbourhood of the melting point of the filament material. I v It has been found that a tungsten wire heated in this way on the one. side develops a pronounced fine grained structure, i. e. the'most frequently occurring or predominating crystal size being a few or several thousandths mm., and

that on the other side it has practically no sag when the lamp is in operation, that it therefore meets with all the requirements relative to the constancy of the form.

A suitable method of eflecting theshock heating is to subject the filament while fixed to its supports to the action of an electric current in the circuit of a transformer having a leakage of over 25%. By employing a transformer with this leakage ensures that the voltage applied will drop byabout or more immediately the circuit is '30 completed andfurthermore it also obviates arcdischarges of the filament during heating.

The same purpose can be achieved by the use of a condenser, with or without an inductance placed in series. The employment of either a transformer or a condenser with inductance enables the desired shape of the temperature curve to be adjusted by the selection of the constants.

l ilaments of coiled tungsten wire produced according to the method described, exhibit a fine grained structure with apredominating crystal size of a few or several thousandths and possess a sag of less than 10% or even less than 5%. The percentage of sag is measured by the percentage relationship ofthe arrow (sagitta) to the chord ofv the segment, the segment being the area between the positions of the filament at the commencement and after burning.

It will be found that a filament manufactured by the method of this invention will at its ends be of a substantially fibrous structure, whereas theremainder i. e. substantially the entire filament is recrystallized. This variation in struc- V ture is due to the cooling uence of the electrodes (current supply wires) durin the heat treatment.

--Astospecialkindsofcoiledtungstenwireitis advisable first to ascertain the suitability of the wire by microscopic inspection of filaments subjected to the herein described shock heat treatment under varied temperatures in the neighbourhood of the melting point.

What I claim is:

1. A process for manufacturing coiled filaments from drawn tungsten wire inclined to the forma-, tion of long crystals suitable for electric incandescentlamps which consists in subjecting the coiled filament for a short time-less than about second, to a shock heating at a temperature in the neighbourhood of the melting temperature I of tungsten.

2. A process for manufacturing coiled filaments from drawn tungsten wire inclined to the formation of long crystals suitable for electric incandescent lamps, which includes previously subjecting the coiled filament on a mandrel of refractory metal to a heating at a temperature of about 1100 C. which lies below the recrystallization temperature and then subjecting the coiled filament for a short time less than about sec-' time being less than about is second, to a shock heating at a temperature which is in the neighbourhood of the melting temperature of tungsten. 4. A process for manufacturing coiled filaments from drawn tungsten wire inclined to the formation of long crystals suitable for electric incandescent lamps which consists in subjecting'the ooiled filament for a short time less than about 5 second, to a shock heating at a temperature which is in the neighbourhood of the melting temperature of tungsten the said heating being eflected by the passage of an electric current in the circuit of an electrical transformer having a leakage of more than 25%.

5. A process for manufacturing coiled filaments from drawn tungsten wire inclined to the formation of long crystals suitable for electric incandescent lamps which consists in subjecting'the coiled filament for a short time less than about 1% second, to a shock heating at a temperature which is in the neighbourhood of the melting temperature of tungsten, the said heating being effected by-the passage of an electrical current in the-circuit of an electrical condenser.

6. A process for manufacturing coiled filaments from drawn-tungsten wire inclined to the formation of long crystals suitable for electric incandescent lamps which consists in subjecting the coiled filament for a short time less than about second, to a shock heating at a temperature which is in the neighbourhood of the melting temperature of tungsten, the said heating being eifected by'the passage of an electrical current in the circuit of an electrical condenser with an inductance in series therewith.

JAKOB SAIPETER.

lies belowthe recrystallization temperature and then subjecting the coiled filament for a short 

